Nuclear accidents and terrorism presents a serious threat for mass casualty. Accidental or intended radiation exposure in a mass casualty setting presents a serious and on-going threat. Acute radiation injury is manifested in organs that have rapidly proliferating cells, such as, intestine, mucosal lining of the body, bone marrow and skin. Manifestation of acute radiation injury includes anemia, bleeding, diarrhea, sepsis, mucosal and cutaneous ulceration and even death due to target organ failure. At radiation doses of 3 to 8 Gy, morbidity and lethality is primarily caused from hematopoietic injury and victims can be rescued by bone marrow transplantation (BMT). However, with exposure to larger doses, victims suffer irreversible hematopoietic and gastrointestinal injury and usually perish despite supportive care and BMT. There are currently no approved treatments to alleviate Acute Radiation Syndrome (ARS) in victims of radiological disaster with anticipated multi-organ failure or to effectively treat/protect first responders from ARS. To date, individuals categorized as H4 (dose and volume irradiation dependent) receive supportive care post-radiation exposure that includes reverse isolation, antibiotics, antivirals, antifungals, platelet and blood transfusions and maintenance of fluid/electrolyte balance prior to bone marrow transplantation (BMT), resulting in only marginal survival. Also, while BMT may have some benefit in mitigating hematopoietic syndrome, currently there are no approved medical countermeasures to alleviate radiation-induced gastrointestinal syndrome (RIGS).
While radioprotective agents can be used with some success when given prior to radiation exposure they are of limited use when used post-exposure. This circumstance motivates the continued search for agents that alleviate radiation damage post-exposure.
Late radiation injury is manifested in organs that have parenchymal cells that divide slowly, such as, brain, spinal cord and liver. In addition, chronic radiation injury can occur in any organ, including lung, kidney, intestine, esophagus, bladder and rectum. Chronic radiation injury is caused by aberrant repair of acute radiation injury and is usually seen as a fibrotic response.
Syndromes and symptoms that are caused by radiation injury include xerostomia (dry mouth), dysphagia (difficulty in swallowing) due to pharyngeal and esophageal strictures, breast fibrosis, cutaneous ulcers, dyspnea due to radiation pneumonitis and lung fibrosis, radiation-induced liver damage, kidney failure due to fibrotic kidneys, rectal bleeding due to radiation proctitis, bladder and urethral injury, diarrhea, enteric bleeding and sepsis due to radiation-induced gastrointestinal syndrome, and anemia, thrombocytopenia and neutropenia from radiation-induced marrow failure. Basically most organs can manifest some form of acute or chronic radiation injury.
There are currently there are no approved medical countermeasures to alleviate radiation-induced gastrointestinal syndrome (RIGS), resulting from direct cytocidal effects on intestinal stem cells (ISC) and crypt stromal cells. RIGS results from a dose-dependent, direct cytocidal and growth inhibitory effects of irradiation on the villous enterocytes, crypt intestinal stem cells (ISC)[1,2,3], the stromal endothelial cells[4] and the intestinal subepithelial myofibroblasts (ISEMF)[5]. Subsequent loss of the mucosal barrier results in microbial infection, septic shock and systemic inflammatory response syndrome. The cells in the ISC niche, consisting of micovascular endothelial cells, mesenchyme-derived ISEMF[5] and pericryptal macrophages[6] provide critical growth factor/signals for ISC regeneration and intestinal homeostasis[7]. Of these, ISEMF continuously migrate upward from the crypt base to the villous tip along with ISC and transit amplifying enterocytes, establishing signaling crosstalk and regulating ISC self-renewal and differentiation[5,8]. ISEMF interacts with pericryptal macrophages with subsequent release of PGE2 that could reduce radiation-induced apoptosis of enterocytes[9,10]. Pericryptal macrophages form synapses with crypt stem cells and secretes growth factors to stimulate ISC proliferation[6] upon activation of Toll-like receptors sensing the entry of bacteria and other intestinal pathogens.
Since RIGS results from a combination of radiation-induced loss of crypt progenitors and stromal cells along with aberrant signaling in the ISC niche, it is possible that the acute loss of stromal cells in the ISC niche would require rapid compensation of their functions. This might possibly be best achieved with cell replacement therapies that restore the ISC niche after irradiation so that the stromal cells can secrete growth factors and provide necessary signals for survival, repair and regeneration of the irradiated intestine. Earlier reports demonstrated that donor bone marrow-derived cells could contribute to multiple lineages in the gastrointestinal tract and facilitate intestinal regeneration in patients with graft-versus-host disease and ulcer[11] and in animal models of colitis[12].
Because of ease in cell culture and its ability to differentiate into multiple tissue lineages, transplantation of bone marrow-derived mesenchymal stem cells (MSC) has been a very attractive option for a wide range of clinical applications[13], such as, severe treatment-resistant graft-versus-host diseases of the gut[14]. Besides trans-differentiating into ISEMF and stimulating ISC proliferation, MSC transplantation has also been shown to reprogram host macrophages to induce an anti-inflammatory response and thereby minimizing sepsis in a murine model of colitis[15]. Intravenous injection of MSC resulted in enhanced engraftment in irradiated organs, including, small intestine with subsequent increase in the regeneration of the intestinal epithelium and accelerated recovery of the villi post-radiation in mice models[16]. Genetic modification of donor MSCs with superoxide dismutase[17] or CXCR4[18]transgene augments the engraftment and mitigation of intestinal radiation injury. However, till date, transplantation of whole bone marrow or MSC has not been successful in ameliorating RIGS and improve survival of mice that received >10 Gy of irradiation in a single fraction[16,17,18].
Effective therapies for treating, mitigating or protecting from or preventing injuries associated with exposure to excessive radiation are still needed, and the present invention provides such therapies.